It looks like all Panzer Campaigns games have this one little issue. Why are all German units are rated B+ while Allied ones tend to be around C-D? I can see this working for the Eastern Front, but why'd an American battalion in Normandy or Sicily rated worse than its German counterpart? What is the motive behind this? Gameplay balance?
I think the answer varies from title to title, but I think in general the German forces man for man outperformed the Allied forces for much of the war and it was only by 44 with their manpower dwindling and being diluted by inferior replacements that the Allies achieved parity, if you look at MG44 for example you will see that there is a whole mix of German morale ratings from A to E, while in early war titles they tend to be all A and B.
On defence the Germans were stubborn SOB's and rating them as A can help to reflect that, this approach was used for example in S43.